Fix broken unit tests

My previous commit broke a couple of unit tests. This fixes them.

Author: Ignacio Quintero

src/sagemaker/amazon/hyperparameter.py

@@ -52,7 +52,7 @@ def __get__(self, obj, objtype):
 
     def __set__(self, obj, value):
         """Validate the supplied value and set this hyperparameter to value"""
-        value = self.data_type(value)
+        value = None if value is None else self.data_type(value)
         self.validate(value)
         if '_hyperparameters' not in dir(obj):
             obj._hyperparameters = dict()

tests/unit/test_hyperparameter.py

@@ -16,7 +16,7 @@
 
 class Test(object):
 
-    blank = Hyperparameter(name="some-name")
+    blank = Hyperparameter(name="some-name", data_type=int)
     elizabeth = Hyperparameter(name='elizabeth')
     validated = Hyperparameter(name="validated", validate=lambda value: value > 55, data_type=int)